University of ZagrebFaculty of Mining, Geology and Petroleum Engineering

Place

Zagreb

State

Croatia

Scientific field, discipline, subdiscipline

TECHNICAL SCIENCESMining, Petroleum and Geology EngineeringMining

UDK

622APPLIED SCIENCES. MEDICINE. TECHNOLOGYMining

Study programme type

university

Study level

postgraduate

Study programme

Applied Geosciences, Mining and Petroleum Engineering

Academic title abbreviation

dr. sc.

Genre

doctoral thesis

Language

Croatian

Defense date

2016-09-30

Promoted date

2017-05-07

Parallel abstract (English)

Geosynthetic clay liners (GCLs) are factory-manufactured hydraulic barriers consisting of
mineral (bentonite) and geosynthetic components (geomembrane or geotextile).
The purpose of the study was to identify the extent of change in bentonite clay properties
when built into the final cover of the landfill brought about by the processes taking place
within the landfill body and in its immediate surroundings.
Analyses were made on a total of 17 samples of the cover soil, 24 samples of bentonite clay
and 10 samples of GCLs. 23 samples of bentonite clay and samples of the cover soil were
collected from the sides of the final cover system of a landfill in central Croatia into which
GCL had been built over a period of three to nine years. One sample was not built into the
landfill, which is why all the values obtained by its analysis are presented as a reference
value. Likewise, nine GCL samples were collected from the landfill, while the results of the
sample which wasn’t built into the landfill represent a reference value.
Analysing the cover soil, the values of its depth, moisture content, density and grain size
distribution were obtained. The results obtained from physical, chemical and mineralogical
analyses and analyses of index properties of bentonite clay show that compared to the
reference sample the properties of the sampled bentonite clay have changed during the long
period spent in the landfill. In addition to that, hydraulic conductivity was analysed in order to
investigate whether and to what extent the efficiency of GCL as a hydraulic barrier has
decreased and to investigate whether there is a correlation between index properties and
hydraulic conductivity.
It was identified that the change of bentonite clay properties is affected by the quality of
construction of the final cover system, primarily the depth and moisture content of the
overburden and its grain size distribution, and the year when the sample was built in, i.e. the
period during which the bentonite clay has been present in the landfill. In addition to that, the
properties change due to physical, chemical and mineralogical composition of bentonite clay,
and the conditions taking place in the landfill body.
The analyses not only prove that bentonite clay properties (swelling, absorption, fluid loss
index) have degraded during long-time use in landfills, but also show that both the mean
values of a specific index property and deviations from the mean value vary depending on the
year of building in. Analyses show that the samples built in during 2004 have relatively
consistent properties; stronger deviations are noticed in the samples built in during 2010;
while the largest deviations are present in the samples built in during 2009. This was the
reason why it was not possible to establish a unique correlation between the change in
properties and the time of construction. However, it has been identified that the potential
cause for such inconsistency is probably an uneven composition of bentonite clay used during
the manufacture of GCL built into the landfill. Despite that, the changes that had taken place
in bentonite clay during the time elapsed since construction are not substantial, which is why
bentonite clay and consequently GCL show a still satisfactory sealing property in accordance
with the recommendations applied for barriers in the final cover system. A more significant
change of hydraulic properties of GCL has been observed only in a few locations and was as a
rule associated with an inadequate protective layer, either its depth or physical properties.
Thus has led to more intensive drying-out and stronger change in hydraulic properties of
GCL, which grew more marked as consolidation stress decreased.
Furthermore, the paper also addresses interdependencies (correlations) between index
properties of bentonite clay and the hydraulic conductivity of GCL. Since hydraulic
conductivity is the most important parameter in the design of hydraulic barriers, this provides
a simple and fast procedure to detect potentially problematic zones and assess long-term
efficiency of GCLs based on implementation of a bigger number of simple and cost-effective
tests.